Freeze DodoSSH.Contracts v0.1 (M1)

The second M1 gate. This assembly, not the OpenAPI document, is the client's contract,
so PublicApiAnalyzers now tracks all 540 public members: a renamed DTO property becomes
a build error rather than a runtime deserialisation failure on someone's laptop.

Contract surface:
- EncryptedPayload carries the envelope plus the KeyGeneration and AadVersion columns
  needed to recompute AAD, since AAD is derived from the row rather than transmitted.
- Sync: push with per-operation status (Applied/Conflict/Forbidden/Invalid/Duplicate) so
  one stale item cannot block a whole offline queue; a Conflict returns the server's row
  for client-side three-way merge, because the server cannot merge ciphertext.
- Enrollment: KeyStatement whose hash becomes the OIDC nonce, so the identity provider
  signs over the public keys and this server cannot fabricate a key for a user who never
  enrolled.
- Meta and .well-known configuration: capability negotiation instead of URL versioning,
  which is what a self-hosted product needs when client and server upgrade independently.
- SyncPlaintextFields deliberately has no label or name field. ACL admin runs client-side
  where names can be decrypted, so the server never needs a searchable title.

Two design problems found by writing the tests rather than assuming:
- Hand-constructing JsonSerializerOptions and merely pointing its resolver at the context
  silently discards every source-generated setting. JsonSerializerDefaults.Web replaces
  NumberHandling.Strict with AllowReadingFromString, so "1" would be accepted where 1 is
  meant — invisible until two implementations disagree. Callers now use ResponseOptions or
  StrictRequestOptions; StrictRequestOptions is derived by copying so it cannot drift.
- StrictRequestOptions had a static-initialisation cycle: it read the generated Default
  property from the same type's initialiser and got null. Now lazy.

Requests reject unmapped members so a client typo is a 400; responses tolerate them so an
older client can read a newer server. Enums cross the wire as strings, so reordering one
cannot silently reinterpret stored data.

Also: excluded source-generator output from PublicApiAnalyzers. The JSON generator emits a
public member per serialisable type, which would have added hundreds of mechanical entries
and drowned the ones describing the actual wire contract. And disabled MA0048's
one-type-per-file rule: splitting SyncPullRequest from SyncPullResponse makes a reviewer
open two files to understand one endpoint.

Verified: 0 warnings, 95 tests pass, format clean.

src/DodoSSH.Contracts/SyncEntityType.cs

@@ -0,0 +1,45 @@
+namespace DodoSSH.Contracts;
+
+/// <summary>
+/// The kind of vault item a sync change refers to.
+/// </summary>
+/// <remarks>
+/// Values are persisted in the change log and are part of the wire contract. Append only;
+/// never renumber. These mirror <c>CryptoSpec.AadResourceType</c> for the item types, but are
+/// a separate enum because only syncable items appear here.
+/// </remarks>
+public enum SyncEntityType
+{
+    /// <summary>Not a legal value.</summary>
+    Unspecified = 0,
+
+    /// <summary>An SSH host.</summary>
+    Host = 1,
+
+    /// <summary>A credential.</summary>
+    Credential = 2,
+
+    /// <summary>An SSH key pair.</summary>
+    SshKey = 3,
+
+    /// <summary>A host group.</summary>
+    HostGroup = 4,
+
+    /// <summary>A tag.</summary>
+    Tag = 5,
+
+    /// <summary>A host-to-tag association.</summary>
+    HostTag = 6,
+
+    /// <summary>A host-to-credential association.</summary>
+    HostCredential = 7,
+
+    /// <summary>A command snippet.</summary>
+    Snippet = 8,
+
+    /// <summary>A port forward.</summary>
+    PortForward = 9,
+
+    /// <summary>A known SSH host key.</summary>
+    KnownHostKey = 10,
+}